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You are updating both the data and the index when updating amount instead of just the data. That is twice the number of IO's.

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It's the same IOs. PG does it regardless. Both the data and index

I believe they're confusing a single index with more columns vs adding additional indexes. Additional indexes would be slower to update.

I suppose additional columns could be slower to update too (more data structure traversal), but I'm guessing not by nearly as much as an additional index.

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